Many machine parts, components, brackets or supports, mountings, and the like, are made of cast iron. Such cast iron castings have been used because of the strength and durability of cast iron. However, cast iron is not infallible and sometimes such castings will become cracked or broken. In the past, it has been very difficult, or even impossible, to repair such cast iron castings. A number of repair methods have been utilized, each with its own advantages and disadvantages. However, no one method for repairing cast iron castings is appropriate for all repair situations. Generally, the repair methods have included welding, brazing, epoxys, and metal stitching.
The primary drawback to using welding to repair a damaged casting is that dependable results are not consistently obtained. Cast iron is not structurally capable of withstanding concentrated applications of heat incident to the welding repair. A casting is frequently weakened significantly or is rendered brittle in the areas of the heat application. So weakened, it is not unusual for a repaired casting to develop new cracks near the welded area. Also, a casting made brittle by welding is extremely difficult to machine and is not nearly as tolerant of stress as the original casting. Thus, welding repair of castings usually invites recracking.
Similar disadvantages are encountered when brazing is used to repair a broken or cracked casting, because brazing also requires a specific application of heat to the casting and its repair area.
A further disadvantage to using welding or brazing in casting repair is that the casting cannot normally be repaired on-site. This is because the complex methods of welding and brazing require specific equipment that may not be mobile for on-site repairs. Thus, the casting must be removed from the location where it is used and transported to where it may be repaired. This can be extremely expensive and frequently results in significant plant down-time.
Another method for casting repair is the use of epoxys or other chemical adhesives to fill in the cracked or broken portion. However, such repairs are usually only temporary because the bond created will not withstand the stresses which causes the damage to the casting in the first instance or the normal stresses placed on the casting during ordinary use.
Metal stitching has been used when extreme heat may alter or destroy the molecular structure of the casting, as well as when other methods are impractical or prohibitive. With metal stitching, high tensile strength fasteners are embedded across the cracks in the damaged casting to firmly secure the separated elements. Then, tapered lacing plugs are worked into the metal along the cracks providing a pressure and liquid tight seal. Since most metal stitching repairs can be implemented on-site without disassembling the damaged casting, down-time and labor costs are significantly minimized.
However, the present forms of metal stitching do have some drawbacks. At present, metal stitching is performed by pounding a metal lock into a line of bores that have been prepared to receive the lock. The line of bores run transverse to the crack. The metal lock used has a concave curvature and usually comprises three or five lobes of equal diameter aligned such that the circle of each lobe slightly intersects the next adjacent lobe. Because of the concave structure of the metal locks now used for metal stitching, a couple of very significant disadvantages occur during the process of pounding the metal lock into position within the series of intersecting bores prepared to receive the lock. First, it is the outermost lobes of the metal lock which engage the prepared bores first. When these outermost lobes are forced into the prepared bores they can cause the crack to further separate thereby introducing new stresses and strains to the casting. Second, this problem is further aggravated because as the metal lock is forced into position deep within the prepared bores the concave metal lock tends to flair out from its center thereby compounding the stresses which may cause separation of the crack in the casting.
One or more metal locks are usually secured within a set of prepared bores. If the length and nature of the crack require additional constriction, several metal locks are positioned across the crack at intervals of distance to hold the casting and prevent further cracking. When it is desired to maintain a pressure or liquid tight seal in the casting, a series of lacing plugs are typically used to seal the crack from leaking. These lacing plugs are positioned in a slightly overlapping alignment along the length of the entire crack and between each of the metal locks.
The lacing plugs presently being used are tapered plugs with tapered threads. Such lacing plugs are placed within bores prepared along the length and path of the crack. Because a very tight fit is desired, the threads of the tapered lacing plugs are usually slightly oversized. In this manner the plug engages the threads in the bore in pressure and liquid tight engagement. After the plug is tightened into the receiving bore, the head of the plug is cut off and the remaining stub is ground and/or peened flush with the surface. The other tapered plugs are positioned seriatim in a similar fashion along the crack. When the lacing is completed, it runs the full length of the crack and each plug intesects with the next adjacent plugs to assure that a proper seal is obtained.
The tapered lacing plugs known and used in the art have presented several problems which heretofore have remained unsolved. Because the lacing plugs are tapered and have tapered threads, they are designed to threadably engage a receiving bore which has been tapped with a tapered tap. It is the present practice to drill straight bores and then tap that bore with a tapered tap. This is a difficult procedure that frequently results in damaged threads or a broken tap. When a tap breaks within the receiving bore it is extremely difficult and time-consuming to remove the broken tap. Removal of the broken tap almost always damages the treads and damaged threads drastically increase the likelihood that a pressure and air tight fit may not be achieved. Thus, the cost of and time loss attributable to each job requiring lacing is increased.
Another problem with the prior art tapered plugs arises because they are typically slightly oversized to assure a snug fit. Thus, as they are threaded into position, they act like a wedge being driven into the metal. Consequently, with the insertion of each successive tapered plug into the lacing, there is a tendency to loosen the prior inserted plugs and to further separate the crack. In many instances, this increases the stresses on the repaired casting and may ultimately cause further cracking or new cracks in the casting. Also, when the plugs are loosened their effectiveness is significantly reduced. Thus, after the heads of the tapered plugs are removed, the plugs are peened so as to seat the plugs more firmly to prevent leaking that may be caused by the loosened plugs. Such peening damages the surface of the casting. The damaged surface must then be repaired through time-consuming cosmetic repair techniques.
Still another problem involves the actual threading of the plugs into the receiving bores. If for some reason the plug binds during threading, it may break off. Such plugs do not break off cleanly above the surface; rather, they usually break off in an irregular break which is, at least, partially below the surface of the casting being repaired. Unless repaired by inserting another plug, a scar is left in the surface of the casting and this scar is a weak spot in the seal.
A further disadvantage to metal stitching in general is that it has been somewhat limited to smaller applications and applications where the structure of the casting permits drilling at or near the area of the crack or break. There are occasions, however, in which precision drilling in the area of the crack or break cannot be done. In those instances, metal stitching has not been a practical approach to casting repair.